Processes for converting lower alcohols such as methanol to hydrocarbons are known and have become of great interest in recent times because they offer an attractive way of producing liquid hydrocarbon fuels, especially gasoline, from sources which are not of liquid petroliferous origin. In particular, they provide a way by which methanol can be converted to gasoline boiling range products in good yields. The methanol, in turn, may be readily obtained from coal by gasification to synthesis gas and conversion of the synthesis gas to methanol by well-established industrial processes. As an alternative, the methanol may be obtained from natural gas by other conventional processes.
The conversion of methanol to hydrocarbon products may take place in a fluidized bed process as described, for example, in U.S. Pat. Nos. 4,071,573 and 4,138,440, or in a fixed bed as described in U.S. Pat. Nos. 3,998,899, 3,931,349 and 4,035,430. In the fixed bed process, the methanol is usually first subjected to a dehydrating step, using a catalyst such as gamma-alumina, to form an equilibrium mixture of methanol, dimethyl ether (DME) and water. This mixture is then passed over a catalyst such as zeolite ZSM-5 which brings about the conversion to the hydrocarbon products which are mainly in the range of light gas to gasoline. The water may be removed from the methanol dehydration products prior to conversion to hydrocarbons as may the methanol which can be recycled to the dehydration step, as described in U.S. Pat. No. 4,035,430. Removal of the water is desirable because the catalyst may tend to become deactivated by the presence of the water vapor at the reaction temperatures employed, but this step is by no means essential.
The conversion of oxygenates, or more particularly methanol, to gasoline, typically referred to as the MTG process, is highly energy efficient. The hydrocarbons from the conversion contain 95 percent of the energy in the original methanol feed; the other five percent is released as exothermic heat and used during the conversion reaction. Recycling of process gas limits the temperature rise across the fixed catalyst bed to less than 95.degree. C. Also during the reaction, a small amount of hydrocarbon is deposited on the catalyst as coke, requiring periodic catalyst regeneration. Operation of the process, however, is continuous because additional reactors, arranged in parallel, permit an individual reactor to swing from operation to regeneration while another goes from regeneration to operation. The final gasoline yield from the fixed bed process, after alkylating the light olefins formed, is about 85-90 percent by weight of the total hydrocarbons formed. The remaining hydrocarbons are available mostly as liquid petroleum gas (LPG) and a small amount of fuel gas.
The conversion of oxygenates is described in depth by C. D. Chang, Catal. Rev.-Sci. Eng., 25, 1 (1983) and in U.S. Pat. Nos. 3,931,349 to Kuo and 4,404,414 to Penick et al. These references are incorporated herein in their entirety.
Major problems facing research workers in the field of the MTG process include improvements in cycle average yield of gasoline and the extension of catalyst cycle life. Improvements in yield and catalyst life are known to be inextricably related, whereby advances in one problem area are typically achieved at the expense of the other. Process improvements leading to the common enhancement of gasoline yield and catalyst life have been most elusive. One factor that complicates the effort of research workers to achieve the desired advances in yield and catalyst cycle life is the requirement that the MTG process operate at or near quantitative methanol conversion. Less than quantitative conversions, or "methanol breakthrough," presents severe problems in waste disposal and/or methanol recovery which quickly leads to punishing economic penalties and, therefore, is to be avoided. Accordingly, whatever advances research workers are to make in yield and catalyst life for MTG improvements must be made while maintaining essentially quantitative conversion of methanol.
The MTG process is a gas phase process which produces isoparaffins and aromatics with virtually no oligomer formation. The coke formed in the process is harder and more condensed than that from other processes such as zeolite catalyzed olefin oligomerization. Accordingly, it is much harder to remove. It is, however, well known that deactivated or aged zeolite oxygenate conversion catalyst can be regenerated by contacting the catalyst at elevated temperature with an oxygen-containing gas such as air to effect controlled burning of coke from the deactivated catalyst. While such a conventional regeneration procedure can restore catalytic activity diminished by coke formation in the catalyst during the conversion reaction, regeneration in this manner can lead to catalyst damage requiring more frequent, and expensive, catalyst replacement. There is, therefore, a continuing need to find better methods to regenerate deactivated catalyst in fixed beds in order to lengthen time on stream, or cycle length.
Accordingly, it is an object of the present invention to provide a more effective and non-oxidizing process for the reactivation of a fixed bed containing spent or deactivated zeolite catalyst.
Another object of the invention is to provide a catalyst reactivation method which would extend the cycle time between regeneration of fixed bed zeolite catalyst compared to conventional reactivation methods.
Yet another object of the present invention is to provide a process more effective and useful for the reactivation of deactivated zeolite catalyst for lower oxygenate conversion processes.